Use of light to denature very specific kinds of tissue has been called wavelength-selective photo-thermolysis. The use of lasers for this purpose has been well described in the literature. See, for example, R. G. Wheland, “Laser-assisted hair removal”, Lasers in Dermatology, Vol. 15, pp. 469-477, and references cited. By choosing a laser with the right wavelength and energy per unit area (fluence), a particular light-absorbing target substance (chromophore) in living tissue, such as melanin or possibly hemoglobin, will absorb energy from the treatment beam and become hot enough to destroy functionality in the tissue containing the chromophore. Tissue in the same area that does not have high concentration of the target chromophore will not be as affected.
Hair H, see FIG. 1C, includes two basic parts, the shaft S, which is the portion of the hair above the epidermis E, and the root R, which is the portion below the surface of the epidermis. Various tissues surround the root of the hair. Hair color is primarily due to the presence of melanin in the hair. Melanin is created at the base of the hair follicle F and is passed into the hair as it grows. The presence of melanin has made it possible to use certain lasers and other light sources for hair removal with melanin as the target chromophore. The hair follicle and surrounding structure (referred to collectively as hair tissue T) are selectively heated when the melanin in the hair tissue and in the hair root itself and is exposed to treatment radiation. The hair tissue is thermally damaged as a result of the localized heating after treatment; many of the exposed hairs atrophy and are sloughed from the epidermis.
The early work in this field was centered around a wavelength with very high melanin absorption, the pulsed ruby laser (694 nm). Long pulse ruby lasers (as opposed to Q-switched ruby lasers) typically have a pulse duration, or a series of pulses, in the 1 millisecond range. Although the wavelength is highly absorbed in melanin, the wavelength selection has significant limitations with darker skin types as the epidermis can blister or induce a pigmentary change from the superficial melanin heating.
Dermatologists have used cooling devices in dermatologic applications prior to laser treatment. The purpose is to chill the skin with the understanding that exposure to treatment radiation will elevate the epidermal temperature. Chilling lowers the initial temperature so that the post treatment temperature at the epidermis will not create a heat-induced blister or pigmentary change. U.S. Pat. No. 5,735,844 describes apparatus which uses a cooled lens, through which radiation passes, pressed against the patient's skin to cool the epidermis.
Many different approaches to hair removal have been explored since the early ruby laser evaluation. A common trend is a continual shift towards longer wavelengths, which have less melanin absorption, as it allows treatment of patients with a darker range of skin tones. Initially, alexandrite (755 nm) was evaluated and later a diode approach (810 nm). The alexandrite laser offers some improved clinical capabilities over the ruby laser if one considers treatment of darker skin types. However, from engineering and system performance measures, the two systems are similar in terms of size, utility requirement, treatment speed, and system cost. In contrast, the high pulse energy diode laser allows the system to be much smaller than previous systems with an ability to run off of standard power. A high pulse energy diode has many attractive attributes over other laser approaches, such as flashlamp excited solid-state devices and therapeutic filtered flashlamps. One of the most desirable traits is the high efficiency associated with diode generated laser radiation. As a result of improved efficiency, the system can be smaller and lighter weight.
One of the performance limitations working against a diode laser for an application like hair removal is the need for high peak power for short bursts. In general, diode lasers may be pulsed at peak power levels higher than the rated continuous power. However, the peak rating is not a significantly higher power level (perhaps two to three times) than the continuous level. A constraint that a hair removal application has is the need to deliver significant energy deep into the dermis to sufficiently heat the hair tissue to induce a thermal injury so that the hair will slough off. Light scatters as it propagates through the tissue. As a result of this severe exposure, large area treatments (greater than 20 mm2) have been explored to overcome the reduction (due to scatter) in intensity as the light travels deeper into the dermis. A large treatment area can generate a fairly homogeneous intensity profile under the center of the treatment area as light propagates into the dermis.
A commercially-available diode laser system for hair removal is sold by Coherent of Santa Clara, Calif. as Lightsheer and allows the physician to treat the darker skin types than ruby lasers with a lower risk of post operative blistering. The Lightsheer system employs many diode bars (between 50 to 100) to create high peak power (2-4 kW) and a large square treatment area (about 0.81 cm2). The Lightsheer system is used in pulsed operation with treatment rates up to about 1.6 cm2/sec. The system has a pulse duration between 5 and 30 ms, which closely matches the thermal relaxation time of typical hair follicles.